WO2004105602A1

WO2004105602A1 - Method for measuring water content of subcutaneous fat and apparatus for applying of the method

Info

Publication number: WO2004105602A1
Application number: PCT/FI2004/000311
Authority: WO
Inventors: Esko Alanen; David Laaksonen; Tapani Aulis Lahtinen; Leo Niskanen; Jouni Nuutinen
Original assignee: Delfin Technologies Ltd
Priority date: 2003-05-28
Filing date: 2004-05-24
Publication date: 2004-12-09
Also published as: FI20030806A0

Abstract

The invention relates to a method for measuring the water content of the subcutaneous fat tissue in which method an electromagnetic probe (24) is during measurement placed on the skin. The capacitance of the probe at high frequencies like 20-500 MHz is proportional to the dielectric constant of the skin and the subcutaneous fat, which is proportional to the water content of the skin and the subcutaneous fat. Further the invention relates to an apparatus with an electromagnetic probe (24) with the electrodes (30 and 32) operating at the frequency 20-500 MHz. According to the method the dimensions of the probe are set in a way that the water content under the skin may be measured, so the distance between the electrodes (30 and 32) is set to be large, approximately 10-50 mm, in which case the effect of skin on the measured value is small and hence the measured value is proportional to the dielectric constant of the subcutaneous fat tissue and further to its water content. It is characteristic to an apparatus according to the invention that the distance between the electrodes (30 and 32) is large, approximately 10-50 mm.

Description

METHOD FOR MEASURING WATER CONTENT OF SUBCUTANEOUS FAT AND APPARATUS FOR APPLYING OF THE METHOD

The present invention relates to a method for measuring water content of subcutaneous fat in which method an electromagnetic probe is placed on the skin while performing the measurement. The capacitance of the probe, at a high frequency like 20-500 MHz, is proportional to the dielectric constant of the skin and subcutaneous fat tissue, which is further proportional to the water content of the skin and subcutaneous fat tissue. In addition the present invention relates to an apparatus for applying the method. The apparatus consists of an electromagnetic probe with electrodes that is placed on the skin and which operates on a frequency of approximately 20-500 MHz.

The skin is a layer-structured organ with a high-cellular epidermis at the top of the skin. Below the epidermis is the dermis whose blood circulation is controlled by upper and lower vascular plexus. The subcutaneous fat lies below the dermis.

Physical activity and body temperature regulate the blood circulation of the skin and subcutaneous fat. Similarly it can be regulated by drugs and stimulants like alcohol, tobacco and coffee. With an obese person the blood flow of the adipose tissue has been decreased compared to a person with normal weight. Physical activity and weight loss improve blood flow and further the metabolism of the subcutaneous fat tissue.

Insulin that is produced by the body plays a key role in regulating blood flow and metabolism. The metabolic syndrome of obese people is often characterised by hypertension, disturbed glucose metabolism and insulin resistance. On the other hand, weight loss improves insulin sensitivity of the organism and it has benefits described above for blood flow and metabolism in the adipose tissue.

In the accomplished study the measured insulin sensitivity increased statistically significantly during the weight loss period and it persisted throughout the weight maintenance period of one year. In the same study and at the same time the dielectric constant of the subcutaneous fat was measured. It increased during the follow-up period. The dielectric constant at the 300 MHz measuring frequency in the study is proportional to the water content of the tissue.

The measured and increased dielectric constant in the subcutaneous tissue that is the increased water content is related to increased blood flow. Normally the subcutaneous fat is a low- water content tissue (water content about 10-15 weight- %) and the changes in the water content of the subcutaneous fat can be observed through the skin by choosing the dimensions of the measuring probe and the measurement frequency correctly. The increased blood flow of the subcutaneous fat tissue is a desirable condition that helps in weight loss. It can be beneficial to measure it when different kinds of weight loss methods are evaluated.

The water content of most human tissues, like muscle tissue, is normally 72-74 % (Am J Clin Nutr 46, 1987, Lukaski H.C. "Methods for assessment of human body composition: traditional and new", pp. 537-556). The adipose tissue instead has low water content but it varies more than that of other tissues. The so called TBW-index (total body water) can be determined by using isotopes of hydrogen, deuterium and tritium that dilute into the water volume of the body. A disadvantage in using radioactive tracers such as tritium is that their use is forbidden with children or women of child-bearing age. The technique also requires special devices for isotope detection.

Another method to directly assess the water content of adipose tissue is traditional magnetic resonance imaging (MRI). Some of its applications have been described in patents US-6147492 and EP-0851236. Water content can be determined from any part of the body. The method requires an MRI-unit and is too expensive for routine use.

The so-called bioelectrical impedance method determines the total body impedance by using a tetrapolar method where four electrodes are positioned on the upper and lower limbs. The measurement gives an approximation of the total body water content but the method is inaccurate (J Ren Nutr 9, 1999, Di Iorio B.R., Terracciano V., and Bellizzi V., "Bioelectrical impedance measurement: errors and artifacts", pp. 192-197). No local information may be obtained with the method.

The international patent publication WO-02/080770 describes a method for measuring tissue edema with an electromagnetic probe that is placed on the skin. The method utilises the measurement of the dielectric constant of the skin. If tissue edema appears on the skin the dielectric constant of the skin increases for two reasons. Firstly, the edema increases skin thickness and secondly the fat tissue with low water content moves farther away from the probe. In this case the skin with high water content is measured more effectively and hence the measured value is higher. On the other hand also the water content of the skin increases which in turn increases the measured dielectric constant. The essential fact in the method described in the patent publication WO-02/080770 is the size of the measuring probe, which is so small that the measurement applies mainly to the skin. For reasons described above the method of the patent publication WO-02/080770 can not be applied to determine water content of the subcutaneous fat tissue.

The water content of tissue is proportional to the dielectric constant at a high radio frequency. According to well-known techniques the dielectric constant of biological tissues has been measured with electrodes placed inside the tissue. The benefit of these methods is the close contact of the electrodes with the target volume. The measurement is made by sending an oscillating electromagnetic field into the tissue. From the interaction of the electric field and the tissue the dielectric properties of the tissue can be calculated as a function of the frequency. The result of the dielectric measurement is usually a value measured by one or more frequencies. It is proportional to the complex permittivity, dielectric constant or conductivity of the tissue. The disadvantage of these techniques is that the electrodes, usually 2-4, have to be placed invasively into the tissue, hence damaging the tissue.

The object of the present invention is to provide a method and an apparatus, which obviates the shortcomings described above of the current systems. Furthermore the object of the invention is to provide an advantageous method and apparatus for measuring non-invasively the local water content of subcutaneous fat directly from the skin surface of a person. It is a further objective to provide a method an apparatus, which does not impose any restrictions on the measurement site or its water content.

The object of the invention is achieved by the method and apparatus, which is described in the claims.

In a method in accordance with the invention the dimensions of the measuring probe are adjusted so that the water content can be measured below the skin layer. Hence the distance between the electrodes is so big, approximately 10-50 mm , that the relative effect of the skin layer is small and the result is proportional to the dielectric constant of subcutaneous fat tissue and furthermore its water content.

It is an essential feature of the invention to adjust the dimensions of the probe so that the measurement from the subcutaneous fat is possible. The distance between the electrodes has to be so big that the relative effect of the skin is low. A possible choice for this dimension is about 10-50 mm. The most advantageous distance between the electrodes is approximately 15-20 mm in which case the measurement is applied to the subcutaneous fat layer and there is hardly any influence on the measurement by the muscle tissue below the fat layer.

Another essential feature of a method in accordance with the present invention is the high radio frequency (20-500 MHz), because at these frequencies the electric field penetrates deeply into the skin and subcutaneous fat tissue. At lower frequencies the electric field stops on the superficial layers of the skin and hence it is not possible to measure the water content changes of deeper structures.

In a method in accordance with the invention a probe is placed on the skin, and an electromagnetic field, with a high frequency (20-500 MHz), is transmitted through the skin especially into the subcutaneous fat tissue. The electric field that reflects back from the tissue is measured. From the reflected field the dielectric constant of the skin can be calculated. The dielectric constant is proportional to the relative water content of the skin and it increases as the water content increases. The dielectric constant of the subcutaneous fat tissue is normally low (approximately 5- 10) because of the low water content of the fat tissue. The dielectric constant of water is 80. Hence even relatively small changes in the water content of the subcutaneous fat can be detected by dielectric measurements.

Substantial benefits are obtained with the present invention. Changes in the water content can be monitored locally from the surface of the skin by placing the electrode on the measuring site. It can be done without any invasive operation. Measurements taken by the apparatus according to the invention do not disturb the physiology of the tissue in any way.

The effects of medical operations, treatments with drugs, physical treatments, excercise or weight loss and maintenance can be evaluated better with a measurement method in accordance with the invention.

In an advantageous application of the invention the measurement is performed manually and it takes only a few seconds. In this way the local water content of the subcutaneous fat can be rapidly detected.

In a further advantageous application of the invention the device operates only on one exactly pre-selected frequency. Because the electrical properties of a tissue are dependent on the frequency, reliable and comparable information from the tissue can be obtained by measuring with only one pre-selected frequency. The invention will now be described in greater detail with reference to the accompanying drawings, where

Fig. 1 shows a block diagram showing the operation of a device,

Fig. 2 shows the probe connected to the electronic unit by a coaxial cable, and

Fig 3 illustrates the changes in the dielectric constant that is proportional to water content of the subcutaneous fat tissue, during a 9 weeks weight loss period and after that during the weight maintenance period.

Figure 1 shows the block diagram of a device with an oscillator 20, an attenuator 21, a power splitter 22, a directional coupler 23, a probe 24, amplifiers 25 and 26, a phase detector 27, a low pass filter 28 and a digital electronic unit 29.

Figure 2 shows the probe 24, including an inner electrode 30, a Teflon insulator 31, an outer electrode 32, a coaxial cable 33 and an electronic unit 34 comprising the components of Fig. 1 excluding the probe 24. In this application the probe is an open-ended coaxial cable. The distance between the inner electrode 30 and the outer electrode 32 can be adjusted for different applications, in this application it is 15-20 mm. In other applications it can be 10-50 mm.

The block diagram in Fig. 1 is only one application of a device according to the method. It may also be utilised in other ways.

The device operates so that the electromagnetic high frequency (20-500 MHz) signal from the oscillator 20 is led through the attenuator 21, power splitter 22 and directional coupler 23 to the probe 24 and from there to the site to be measured. From the measured site the signal reflects back. Part of this reflected signal is led through the directional coupler 23 to the amplifier 26 and further to one input of the phase detector 27. The signal coming straight from the oscillator 20 is led through the power splitter 22 and the amplifier 26 to the other input of the phase detector 27. The output from the phase detector 27 is led to the low pass filter 28, whose output is a DC voltage proportional to the capacitance of the probe 24. This voltage is further led to the digital electronic unit, where it is AD-converted, scaled and recorded.

The output of the phase detector 27 after the low pass filtering is proportional to the phase difference, which is only dependent on the capacitance of the probe 24. The device operates on a single precisely set frequency and therefore the result is only dependent on the dielectric properties of the tissue and not on conductivity.

The probe 24 is connected to the directional coupler 23 via the coaxial cable so that the signal is connected to the inner conductor of the cable and further to the inner electrode 30 of the probe 24, and the ground signal is connected to the outer conductor of the cable and further to the outer electrode 32 of the probe 24.

Fig. 1 shows only one example of the high frequency implementation of the invention. It is made using known techniques. The essential feature is that the capacitance of the probe is measured at a high frequency 20-500 MHz.

The high frequency unit of the device, comprising parts 20-27 in figure 1, is realised using standard radio techniques. In practice this means that the components are connected to each other with microstrip lines, which have a defined wave impedance, for instance 50 ohms. Therefore the same signal line can propagate signals in both directions simultaneously. In this context the signal means such a propagating wave. The fact that the dimensions of the circuit are small compared to the wavelength does not in any way affect the operation of the high frequency components.

An essential feature of the high frequency unit is that the signal voltage in both inputs of the phase detector 27 is so high that the detector operates in a saturated state. Therefore the phase detector 27 measures only the phase difference of incoming signals. This phase difference is proportional to the capacitance of the probe 24, which is further proportional to the dielectric constant of the tissue. The dielectric constant is proportional on the water content of the tissue.

Another essential feature of the high frequency unit is the attenuator 21 between the oscillator 20 and the power splitter 22. Its purpose is to prevent the access of the signal reflected from the probe 24 to the amplifier 25 input. The signal reflected from the probe goes twice through the attenuator 21 when propagating to the input of the amplifier 25. Therefore, if the attenuation of the attenuator 21 is for instance 6 dB, the total attenuation of this signal is 12 dB, which is adequate.

Fig. 3 shows as an example the change of the dielectric constant proportional to the water content of fat tissue during the weight loss and the following weight maintenance period. It can be seen that the dielectric constant proportional to the water content of the fat tissue increases during the 9 week period of weight loss from the level 23,3 to 25,0, and further to the level 27,8 during the one-year weight maintenance period. Measurements were done with 27 test persons from the abdominal skin. Results are presented as mean values with ± 1 SD. The detected changes are statistically significant.

The structure of the probe can also be other than an open-ended coaxial cable. The present invention is not restricted to the aforementioned advantageous application, but can be utilised in other forms within the limits of the idea of the invention as defined by the claims.

Claims

1. A method for measuring the water content of the subcutaneous fat tissue, where an electromagnetic probe 24 is placed on the skin during the measurement, and the capacitance of the probe, at high radiofrequency 50-500 MHz, is proportional to the dielectric constant of the skin and subcutaneous fat tissue, which is further proportional to the water content of the skin and subcutaneous fat tissue, characterized in, that the dimensions of the probe are adjusted so that the water content is measured under the skin, meaning that the distance between the electrodes 30 and 32 is large, 10-50 mm, in which case the skin effects only slightly to the measured value and hence the measured value is proportional to the water content of the subcutaneous fat tissue and further to its water content.

2. A method according to claim 1, characterized in, that the distance between the electrodes 30 and 32 of the probe is set tol5-20 mm.

3. A method according to claim 1, characterized in, that the measurement is performed manually and takes only a few seconds.

4. A method according to claim 1, characterized in, that the water content of the subcutaneous fat tissue is measured locally.

5. A method according to any of the claims 1-4, characterized in, that the device operates only on a single precisely set frequency.

6. An apparatus for measuring the water content of the subcutaneous fat tissue comprising an electromagnetic probe 24 to be placed on the skin whose operating frequency is 20-500 MHz and electrodes 30 and 32, characterized in, that the distance between the electrodes 30 and 32 is large, approximately 10-50 mm.

7. An apparatus according to claim 6, characterized in, that the distance between the electrodes 30 and 32 is approximately 15-20 mm.